The present invention relates in general to devices that provide a visual display of the value of a generally continually varying function.
There are numerous applications for the visual display of numerical values. In particular, values of continuous or near-continuous mathematical functions often have to be displayed. Sound or voltage levels are examples of functions possibly having occasional discontinuities. Examples of devices used for the display of function values include speedometers, audio signal level meters, flowmeters, and voltmeters.
Mechanical devices, such as a meter using a needle and scale, have long been used for such displays. More recently, electronic devices with digital displays have been used. These usually employ integrated solid state electronics. The digital displays may show numerals representing the function value, or may show a simulation of an analog display (as with a bar display which can grow and shrink, or a display of a simulated indicator needle moving along a scale).
The electronic digital display (EDD devices have a number of advantages over the mechanical device such as cost, size, accuracy, readability, etc.
As a result of the various advantages, the EDD has been replacing the mechanical display in many applications. One highly visible place where this change is happening is on automobile dashboards, involving various readings, including car speed. The auto speedometer will be used as an example herein.
One obvious method of display of a quantity is a display of numerals, usually in decimal digits using the common seven-segment format for each digit. In a speedometer, a decimal number, say "43", would be displayed to indicate speed; this number changes as the speed changes. If higher precision is wanted, a decimal point and fraction digits may be added, such as "43.7".
There are a number of problems with this numeral display. First, a viewer glancing at the display must mentally decode the digits to understand the value displayed. This mental interpretation step may take a fraction of a second more time than viewing an analog needle or bar display. While this is a short time, it may be critical in some circumstances. With the analog display, there is repeated viewing against the same scale, so the scale does not require repeated decoding. One only needs to observe the change of the analog quantity, as through the needle movement or bar's size change.
Second, low-order digits may be changing rapidly, and may therefore be difficult to read. This is especially true when higher precision adds low-order digits, such as fraction digits.
Third, when the speed is just between two display values e.g. 99 and 100, the display may flicker between the two, again making the value hard or impossible to read correctly.
The flickering can be a distracting visual flashing. For example, five of seven segments are changing in a transition between a "1" and a "2" digit, and several digits may be changing simultaneously. This can be dangerous, as in the case of driving an automobile. An appropriate rate of flashing may even induce "flicker vertigo" in some people, and this can produce a trance-like state.
One approach to reduce the digit-changing and flicker problem is to build a delay into the display logic. This forces a display value to persist for a minimum period before a new value is displayed. The difficulty with this is that it prevents accurate display of current values. That is, there is a tradeoff between the visual stability of the display and it's currency.
A method that avoids the problems of numeral display is to use digital components to simulate the appearance of an analog meter. For example, a bar display, made with numerous display segments arranged in picket fence fashion that can be activated to produce a bar of varying length, can be used in conjunection with a scale. This pseudo-analog display allows rapid viewing, as compared to the digit decoding required in the above numeral display.
There are, however, difficulties with this bar display. First, it must be large enough to show the entire scale, as with the analog meter it simulates. This means more space is required for the display, it will weigh more, and in general the cost will be higher for the greater quantity of materials in the device. Second, the more precision is used to achieve a smooth simulation of an analog display, as with the number of segments in the bar display, the more costly the device will be.
One response to the problems of a full simulation of a mechanical meter with an EDD is to break the full scale into a set of adjacent subscales, only one of which is displayed at the time. For example, a 0 to 99 unit speedometer might be split into five subscales of twenty units each: 0-19, 20-39, etc. At any instant, the EDD display would select the subscale required to display the current input value, produce the appropriate display of the subscale, and display a simulated bar against the scale so as to show the current speed value.
A problem with this form of EDD is that if the input value hovers right between two subscales, there may be flashing between the two adjacent subscales. The above discussion of the flashing problem argues strongly against this form of display.
It is possible to retain the advantages of an EDD that uses subscales, yet avoids the flashing display problem and the present invention accomplishes this through the use of overlapping subscales. The display logic of the invention selects subscales so that it is impossible to have a situation where the input can hover near some value and force a flashing back and forth between subscales, as in the preceeding case.
Accordingly, it is an object of the present invention to provide an improved electronic digital display device responsive to a function that may be continually varying and that avoids certain disadvantages of prior art digital display devices.
Another object of the present invention is to provide a digital display device in which a subrange of the full range of values is displayed and the subrange display is appropriately changed with a change in value of the measured function.
A further object of the present invention is to provide an improved electronic digital display device which displays subranges of the full scale in which subrange changeovers cannot produce flashing multiple transitions, regardless of inputs and there is a smooth non-flashing transition between subrange changeovers.
These and other advantages of the present invention will become apparent from the following description of detailed embodiments thereof.